A restricted polyhedral rearrangement of an aminosubstituted 12-vertex ferratricarbollide

The oxidation of anion [7,8-CH2OCH2-7,8-C2B9H10]− with aqueous FeCl3 gives the 10-vertex nido-carborane 5,6-CH2OCH2-5,6-C2B8H10 in 23% yield. Its interaction with ButNC in the presence of proton sponge gives the tricarbollide anion [7,8-CH2OCH2-9-ButNH-7,8,9-C3B8H8]− (44% yield) having a short linkage between carbon atoms. Further photochemical reaction of this anion with [CpFe(C6H6)]+ is accompanied by room-temperature polyhedral rearrangement giving ferratricarbollide 1-Cp-2,3-CH2OCH2-9-ButNH-1,2,3,9-FeC3B8H8 (5) in 89% yield. The process involves the migration of the amino-substituted carbon atom, while the separation of two other carbons (observed for the non-linked analogue) is restricted by the CH2OCH2 bridge. DFT calculations of the hypothetical non-rearranged isomer 1-Cp-2-ButNH-1,2,3,4-FeC3B8H10 revealed its strongly distorted geometry with the C2–C3 distance (2.347 Å) being clearly non-bonding, thus explaining the mild conditions of the polyhedral rearrangement. The structure of 5 was confirmed by X-ray diffraction.

Complexation of amino-substituted tricarbollide anions with the [CpFe]+ fragment is accompanied by room-temperature polyhedral rearrangement. Mild rearrangement conditions are explained by non-bonding character of the C–C connectivity adjacent to the ButNH substituent in the hypothetical non-rearranged isomer. The presence of the linkage between cage carbon atoms prohibits their separation.Figure optionsDownload as PowerPoint slide